Disk drives often employ voice coil motor driven actuators to move one or more read/write heads over data storage media. The voice coil motor, VCM, is constructed from a voice coil that provides the force necessary to move the actuator. Moving the actuator and thus the read/write heads to a new position is known as a seek.
The voice coil varies a magnetic field in the proximity of a permanent magnet. The magnetic field varies in proportion to the VCM coil current. A microprocessor executes an actuator control program including an estimator that controls the current to the VCM through a current driver. A position error signal provides feedback to the estimator from the actuator. When the disk drive seeks to a new location the controller provides a command to change the motor current. The time required for the current to reach the commanded level is known as the voice coil motor current rise time.
Drive performance is affected by the time required to seek to a new location on the storage media. The faster the seek, the higher the performance of the drive. One of the main problems with executing a fast seek in the present disk drive actuator control systems is the rise time of the voice coil current. Because the design of the actuator VCM requires that a coil of wire be placed in a permanent magnetic field to generate the force required to move the actuator, the coil of wire has inductance which characteristically resists changes in current in the coil. The current driver tries to force the current in the coil to the commanded value. However the inductance of the coil prevents the change in current from happening instantly and an exponential rise in current occurs whose rate depends on the impedance of the coil, the power supply voltage, and the back emf voltage.
In the disk drive the estimator is designed to implement a control law. The estimator, based in part on voice coil motor current, tries to estimate the control effort required to position the actuator to the desired location. In the prior art, the control effort input is not accurate for the estimator because of the rise time of the coil current. With an uncompensated estimator, where the control law does not take into account the voice coil rise time, the effective current actually applied to the actuator is not the calculated value. When the rise time of the coil current constitutes an unmodeled characteristic, prior art control algorithms have assumed that the current driver of the VCM is successful in driving the commanded coil current into the coil instantaneously.
One of the physical effects of changing the coil current is the saturation mode of the current driver. Large current changes cause saturation of the current driver which introduce non-linearity in system response. This saturation may cause control problems.
Systems that model the voice coil motor current based on actuator velocity, such as U.S. Pat. No. 4,914,644 entitled "Disk File Digital Servo Control System with Coil Current Modeling" to Chen et al., do not take into account a magnitude of the change in commanded current required in a sample period. The prior art provides a simple model of rise time that does not take into account the complex physical characteristics of the voice coil motor and bases the current model either on a saturation update or a linear prediction update based on actuator velocity. The prior art also does not base control of the drive current on relative magnitude changes in commanded current.
In the prior art, saturation, rise time, and back emf effects are either ignored entirely or are modeled by a constant, and the control algorithms assume that full current is reached immediately by the power driver. This may be a good approximation for small changes in the requested current. However, during seek, large current changes are required as quickly as possible to minimize the seek and settle times. In this case the saturation, rise time, and back emf effects are significant, giving rise to errors in the coil current estimate. As tracks narrow, the gain applied to the coil current in the control algorithm must increase. Thus errors in the position and velocity calculation become larger proportions of a track. These errors cause longer settle times. The voice coil current rise time also causes errors in the residual signal, the residual signal is the difference between what the estimator predicted the position error signal would be and what was actually measured during the next servo sample period.
What is needed is a hard disk drive with an estimator that reduces error in voice coil current rise time based on the magnitude of the change in commanded current.